The invention mainly relates to a method for permitting the monitoring of the operating state of a composite structure formed from at least two different, superimposed materials, when the structure is subject to a uniform load or force, e.g. due to a pressure applied to its entire surface.
The invention also relates to a preferred application of said method to the monitoring of the operating state of the wall of a reservoir for containing a pressurized fluid (e.g. up to 500 bars).
When a reservoir is intended to contain a pressurized fluid, its wall is constructed in such a way that it is able to withstand the internal pressure exerted by the fluid.
When said mechanical resistance stress is coupled with a requirement for a relative lightness of the reservoir, an advantageous solution consists of making the reservoir wall from two different, superimposed materials. More specifically, a metallic, organic or composite envelope is externally covered with a reinforcement of long fibres embedded in a metal or resin matrix. The reinforcement can in particular be obtained by filamentary winding, followed by a polymerization cycle. The fibres can be wound in the form of a single ring or multidirectionally.
When the wall of a reservoir containing a pressurized fluid is constituted by such a structure, the latter can be damaged over a period of time, which then constitutes a safety hazard. In particular, the mechanical behaviour or strength of the composite reinforcement can decrease in time, under the effect of the use of the reservoir or harmful external effects.
EP-A-546 899 and FR-A-2 688 584 disclose methods making it possible to monitor the intrinsic damage state of a composite structure. These methods more specifically relate to the detection of possible delamination or separation defects between the layers of a composite structure formed from superimposed layers of fibres embedded in a resin matrix. A continuous, polarization-maintaining optical fibre serving as a sensor array is placed either between two layers of the composite structure, or between the actual structure and an associated section.
This optical fibre-based monitoring method is difficult to implement, because it is based on the use of an external optical installation. Therefore monitoring is discontinuous and requires a periodic immobilization of the structure in question.
Thus, such a monitoring method is unsuitable when it is wished to know in real time the damage state of a composite structure, without any immobilization thereof.
The present invention is specifically directed at a method for monitoring the operating state of a composite structure subject to a uniform force, e.g. due to a pressure applied to its entire surface and designed so as to ensure a real time monitoring, without necessitating the immobilization of the structure.
According to the invention, this result is achieved by means of a method for monitoring the state of a composite structure formed from at least two different materials superimposed along a given interface and subject to a uniform force over the entire surface thereof, characterized in that it comprises the following stages:
measurement of a deformation or stress level of the structure produced by said force at the interface,
comparison of the measured deformation or stress level with a reference quantity (or a deformation, pressure level or stress level) of the structure, normally produced by said force.
In a preferred embodiment of the invention, the reference quantity is determined by measuring the deformation or stress level of part of the structure, formed from only one of said materials.
Preferably, the measured stress level or deformation and the reference quantity are compared with a reference curve giving the normal evolution of the deformation or stress level of the structure as a function of the force applied thereto.
The reference curve can in particular be obtained by measuring beforehand the evolution of the deformation or stress level of said compsite structure, in the new state, as a function of the force applied to said new structure.
In this case, there is advantageously a modelling of the reference curve by a second degree equation. This simplifies processing, because only two values corresponding to two parameters of the equation are then stored.
In order to measure the deformation or stress level of the part of the structure formed from a single material, use is made of at least one sensor or transducer, such as a strain gauge or any other deformation or stress transducer installed on said part.
In a comparable manner, measurement advantageously takes place of the deformation at the interface by means of at least one deformation or stress transducer, such as a strain gauge or piezoelectric sensor, installed at the interface of the composite structure.
The method according to the invention is advantageously applied to the monitoring of the state of a composite structure, whose different, superimposed materials comprise a metallic or organic or composite envelope and a reinforcement of long fibres embedded in a resin matrix.
The deformation transducer can then be integrated into the composite structure during the manufacture thereof, before making the reinforcement on the envelope.
In a preferred application of the method, the composite structure forms a reservoir able to contain a pressurized fluid.
The part of the structure formed by a single material is then a partition separating an inner area of the reservoir able to contain the pressurized fluid from an instrumentation box, preferably placed e.g. on the bottom of the reservoir.
In order to carry out a more complete monitoring or inspection of the composite structure, it is possible to advantageously measure at least one other quantity chosen from within the group including the temperature at the interface of the structure, the temperature in the immediate vicinity of the structure, the internal pressure of the reservoir, the possible shock or impact levels suffered by the reservoir, the time elapsed since a previous inspection, the time elapsed since the reservoir entered service, the xe2x80x9cemptyxe2x80x9d and xe2x80x9cfullxe2x80x9d states of the reservoir and the number of reservoir fillings.
In order to carry out these different measurements, use is made of monitoring means, such as sensors and electronic circuits, at least partly installed in the instrumentation box of the reservoir.
The invention also relates to a pressurized fluid reservoir comprising a composite wall formed from an internal envelope and on which is deposited a reinforcement of resin-impregnated, long fibres, superimposed along a given interface, characterized in that at least one deformation or stress transducer is integrated into the composite wall, at the interface between the envelope and the reinforcement or within said reinforcement, in order to permit a monitoring of the state of the wall, by comparison of the deformation or stress measured by the transducer with a reference quantity of the stress level or deformation of the wall, normally produced by the pressure of the fluid.